In the steam reforming process a process gas, i.e. a mixture of a hydrocarbon feedstock and steam, and in some cases also carbon dioxide or other components, is passed at an elevated pressure through catalyst-filled heat exchange tubes disposed with in a vessel and which are externally heated on the shell side by means of a suitable heating medium, generally a hot gas mixture. The heating medium may be a combusting hydrocarbon fuel, a flue gas or the process gas that has passed through the tubes but which has then been subjected to further processing before being used as the heat exchange medium. For example GB 1 578 270 describes a process where a primary reformed gas is subjected to partial oxidation where it is partially combusted with oxygen or air and, in some cases is then passed through a secondary reforming catalyst bed (the process known as secondary reforming). The resultant partially combusted gas, by which term we include secondary reformed gas, is then used as the heat exchange medium, passed into the shell side of the primary reformer to heat the tubes. Where a secondary reformed gas is used as the heat-exchange medium, it normally contains methane, hydrogen, carbon oxides, steam and any gas, such as nitrogen, that is present in the feed and which is inert under the conditions employed. If flue gas is used as the heat exchange medium it typically contains large amounts of carbon oxides, steam and inert gasses.
Heat exchange reformers are typically fabricated from Ni-containing steels. Undesirable side reactions can occur under some conditions on the shell side of heat exchange reformer apparatus that are promoted by nickel and iron in the steel, particularly on the heat exchange tubes. The undesirable side reactions include methanation, shift and carburization reactions. We have realised that these reactions result either directly or indirectly from a catalytic interaction between metals in the steel and carbon monoxide (CO) present in the heat exchange medium. In steam reforming where the heat exchange medium in the primary reformer is the primary reformed gas that has been subjected to further processing, this problem is exacerbated by the desire, for economic reasons, to operate at low steam ratios, i.e. low steam to hydrocarbon ratios, which results in increased CO levels in the reformed gas used as the heat exchange medium in the primary reformer.
Methanation is the conversion of carbon oxides to methane and water, i.e. the reverse of steam reforming and is promoted for example by nickel. The CO reaction is depicted below;CO+3H2→CH4+H2O
The shift reaction is the reaction of carbon monoxide with steam to produce carbon dioxide and hydrogen and is promoted for example by iron. The reaction is depicted below;CO+H2O→CO2+H2 
Both these reactions can reduce the efficiency of the reforming process.
Carburization is believed to be, in part, the formation of metal-carbides on the surface of the steel by reaction of the metal with deposited carbon. The deposited carbon can result from both CO reduction and CO disproportionation reactions. These reactions occur on metal surfaces and may be catalysed by Fe, Ni or Cr. The carbon forming reactions are depicted below;                Reduction: CO+H2→C+H2O        Disproportionation: 2 CO→C+CO2         
The carburization of steels is also known as ‘metal dusting’ and leads to corrosion of the metal surfaces, which may lead for example, to failure of the reformer tube. Increased levels of methane in the process gas may also arise via hydrogenation of the deposited carbon.
Because process efficiency and corrosion are effected by the carbon monoxide reactions it is desirable to reduce the interaction between carbon monoxide (CO) present in the heat exchange medium and metals in the steel on the shell side of reformer apparatus.
It is known that sulphur compounds such as dimethylsulphide added to a process gas in contact with a metal surface may suppress the carburization process, but have the disadvantage that the sulphur species formed are mobile and consequently may poison catalysts in subsequent catalysed process steps.
In WO 00/09441 a reforming process is disclosed wherein corrosion of the shell side of a heat exchange reformer by a high temperature secondary reformed gas used as heat exchange medium may be reduced by introducing a sulphur compound, such as dimethylsulphide into the secondary reformed gas after it leaves the secondary reforming apparatus and before it enters the heat exchange reformer as heat exchange medium. The amount of sulphur compound necessary to obviate such corrosion problems was stated to be such as to give a sulphur content of 0.2-20 ppm by volume in the secondary reformed gas. Because sulphur only binds weakly to the catalytic metal sites, to prevent contamination and deactivation of catalysts in subsequent process steps, the sulphur compounds were removed by passing the secondary reformed gas exiting the heat exchange reformer through a bed of a suitable absorbent for sulphur compounds, such as zinc oxide.
It has been found however that to be effective, higher amounts of sulphur in the heat exchange medium are required. However, increasing the level of sulphur in the heat exchange medium above 20 ppm by volume leads to difficulties with removal using conventional absorbents. Furthermore the provision of sulphur-removing apparatus adds additional cost and complexity to the reforming process.
WO 01/66806 describes a method for preventing nitridation and/or carburization of metal surface by adding a sulphur compound and a lower amount of a phosphorus compound to the process gas in contact with the metal. It is suggested that the phosphorus compound acts to prevent corrosive sulfidation of the metal surfaces that occurs when sulphur compounds are added to prevent the nitridation and/or carburization reactions. In the examples, addition of 2 ppm of phosphorus in the form of phosphorus pentoxide to a gas containing 20 ppm of hydrogen sulphide completely prevented the sulphur compound from adhering to the metal surfaces. However, it was also shown that if the metal surface was subjected to phosphine pre-treatment, nitridation of the metal was not prevented.
Attempts have been made to find alternatives to sulphur compound addition. For example, the carburization reaction may be reduced by using corrosion-resistant alloys prepared by alloying other elements with steel (see H. J. Grabke, Research Disclosure, 37031, 1995/69). Other elements cited included lead (Pb), arsenic (As), antimony (Sb), bismuth (Bi), selenium (Se) and tellurium (Te). Antimony and arsenic alloys in particular were stated as having some effect on carburization however their effectiveness varied. This approach however, is expensive and cannot to be implemented in existing reactors without process shutdown.
Alternatively, the same author proposed that carburization may be reduced by applying a solution or slurry of Pb, As, Sb and Bi compounds to the apparatus however no details were provided on how this may be achieved.
Thus no methods have been disclosed that satisfactorily reduce the methanation, shift and carburization reactions on the shell side of reformer apparatus and in particular where the heat exchange medium for the reformer is the primary reformed gas that has been subjected to a further processing step comprising for example, a partial oxidation and secondary reforming step.